Reinnervation of muscle fiber types in the newborn rat soleus.

We have examined the selectivity of reinnervation of fiber types in rat soleus muscle denervated by crush of the soleus nerve 2 d after birth. The fibers innervated by single, regenerated motor axons were identified by use of glycogen depletion approximately 2 weeks following denervation. The types of fibers were determined by immunohistochemistry employing anti-myosin antibodies and, in some cases, by myofibrillar ATPase staining. Two distinct types of fibers are present in soleus at 2 d and through the next 16 d of normal postnatal development. These fiber types are retained in a denervated muscle for the period of time required for reinnervation. Although 40% of the fibers are lost from the muscle during reinnervation, we find no evidence for interconversion of muscle fiber types. Nonetheless, 10 of the 12 single motor units examined had fiber type compositions that were markedly biased toward one or the other of these 2 types; the bias in these units could not be explained by chance reinnervation. On the basis of the topographical distribution of the muscle fibers in each of these units, the motor axons reinnervated a novel set of fibers. We interpret these findings to mean that neonatal soleus motor neurons reinnervate fiber types in a selective manner. This selective innervation may explain the bias in the fiber type composition of normal motor units during early postnatal development.